Typically, a motor vehicle diesel engine is associated with means for processing its exhaust gases for the purpose of reducing the quantity of pollution rejected into the atmosphere, and in particular the quantity of nitrogen oxides or NOx.
To this end, the engine is generally associated with a NOx trap arranged in its exhaust line and adapted to store such particles in the form of a nitrate on specific storage sites, such as barium, for example.
In order to regenerate the NOx trap, operation of the engine is switched over to a rich mixture so as to release into the exhaust line a sufficient quantity of reducers of the nitrate contained in the trap, e.g. HC and CO. The nitrate is then reduced and desorbed in the form of N2 and the storage sites are released so that they can then again store NOx.
Unfortunately, the storage sites are also capable of storing sulfates when they are exposed to the SO2 generated by the engine from the sulfur contained in the fuel and the lubricating oil of the engine. The trap thus becomes progressively saturated in sulfates, thereby having the effect of reducing its catalytic performance.
It is therefore necessary to purge the trap regularly of sulfates in order to eliminate the sulfates that are stored therein.
Unfortunately, because of the great thermodynamic stability of sulfates, merely switching the engine into rich mode does not suffice to reduce the sulfates. For this purpose, it is also necessary to raise the temperature of the trap up to high temperatures of the order of 700° C.
For this purpose, the NOx trap is generally associated with catalyst-forming means arranged upstream therefrom or integrated in the same medium as the trap. The catalyst-forming means are adapted to burn hydrocarbons coming from the engine and thus generate heat for raising the temperature of the trap.
Typically, for purging sulfate from the NOx trap, the engine is controlled to operate with two types of lean mixture. A first lean mixture is determined so as to perform preliminary raising of the temperature of the catalyst-forming means so that they reach a primed state. Once in their primed state, the catalyst-forming means are capable of burning large quantities of hydrocarbons delivered by the engine when operating with its second lean mixture, thereby enabling a high temperature level to be obtained in the NOx trap.
Unfortunately, although it is necessary to purge the NOx trap of sulfate in order to guarantee some minimum level of catalytic performance therefor, it is known that high temperatures damage the trap irreversibly, since the materials constituting the storage sites are degraded by such high temperatures, which amounts to accelerating the aging of the trap.
It is therefore necessary to control the temperature of the trap in a manner that is appropriate for achieving a compromise between effective sulfate purging and premature aging of the trap.
Furthermore, sulfates are released essentially in the form of SO2 when the mixture is rich in oxygen, and in the form of H2S (a foul-smelling gas) when the mixture is poor in oxygen.
The formulation of a NOx trap can advantageously contain oxygen storage compounds (OSC) that release oxygen when the exhaust gas is poor in oxidizing species, i.e. when the engine switches over from operating in rich mode to operating in lean mode.
However, the oxygen storage capacity of the NOx trap is not unlimited, so the available stored oxygen is used up quickly. Thus, when purging sulfate, once the temperature is high enough to enable sulfates to be released, they are initially desorbed in the form of SO2 and then in the form of H2S.